The prior art has on several occasions suggested preparing pavement-marking tapes with reflective protuberances on their upper surface to improve the visibility of the markings, especially at night and when the roadway is wet. One example is U.S. Pat. No. 4,069,281, which teaches a polymeric sheet material formed with protuberances on its upper surface and with reflective glass beads and/or abrasive particles concentrated on the protuberances. As taught in column 5, lines 16 and 17, the concentration of glass beads or abrasive particles is achieved by cascading those particles more densely over the protuberances than over other parts of the sheet material during manufacture of the sheet material, whereupon the particles apparently become partially embedded in the sheet material. The concentration of particulate matter is desired so that other areas of the pavement marking are left more smooth, whereupon they resist the collection of dirt that would occur around projecting particles and that would discolor the marking.
Another pavement-marking tape, understood to be commercially marketed, comprises a mixture of a polymer like polyvinyl chloride and glass beads formed into a sheet having many protuberances on its upper surface. The glass beads are fully embedded in the finished sheet material and become exposed by wear. Since the side surfaces of the protuberances experience little wear, so that few beads become exposed there, and since the side surfaces of the protuberances are most directly in line with the light rays from the headlights of vehicles traveling on the roadway, reflection from the marking is low.
A different commercial pavement marking comprises a stripe of thermoplastic material applied on the roadway and then embossed to have protuberances on its upper face. Glass beads are mixed into the thermoplastic material before it is applied, and additional glass beads are applied to the stripe as it is laid. The additional glass beads apparently fall onto the stripe indiscriminately, both on the protuberances and between them, leading to the problem of dirt collection noted above.
Another related type of prior art product taught in Canadian Pat. No. 868,524 comprises reflective elements having flat upper and lower surfaces and vertical side walls in which glass beads are partially embedded. Such elements are made by extruding a rod of plastic, embedding glass beads in the sides of the rods, and slicing the rod into thin flat elements. The elements are cascaded onto painted lines, where the top flat surface, which has no glass beads, provides improved daylight appearance, and the glass-bead-covered vertical sides provide retroreflectivity.
All of the above products have important deficiencies: some of them are cumbersome to make and use, some do not provide reliable, bright, long-term reflectivity, and some do not achieve an optimum combination of good daytime appearance and bright nighttime reflectivity.